为什么这里的错误出现在三元运算符中？

Question

The below statement gives error "unexpected type"

int i = 3;
System.out.println( (i==3) ? i+=3 : i-=3);

why this happens?

Answer 1

It's due to operator precedence : the += and -= assignment operators have lower precedence than the ? : ? : ternary conditional expression operator.

There is only one way to interpret the i+=3 because it's followed by a : , namely, as the first branch of the ternary. But the -=3 is ambiguous and resolved according to precedence. Because the ternary has higher precedence, the expression is parsed like this:

((3==3) ? (i+=3) : i) -= 3

which is obviously nonsense, because you cannot assign to the result of an expression.

It works if you add extra parentheses to make the assignments take precedence:

(3==3) ? (i+=3) : (i-=3)

The parentheses around i+=3 are optional but recommended for readability (insofar as this thing is ever going to be readable).